This creates a single implementation of the calculation of the contributions of
walls and surface chemistry to the governing equations for all reactor types.
The name 'H' can mean either the species by that name or the entahlpy
of the reactor, in the case of ConstPressureReactor, and the previous
behavior always returned the index of the enthalpy.
This changes the behavior to preferentially return the species, and
adds alternative names for reactor state variables that are less
likely to generate namespace collisions: 'mass', 'volume',
'int_energy', 'enthalpy', 'temperature', 'distance', 'velocity'. The
single character names are still supported.
Resolves Issue 193.
Like IdealGasReactor, this formulation uses the temperature as a state variable
to improve performance for the common use case of reactors containing ideal gas
mixtures.
